1. Field of the Invention
This invention relates to the gasification of coal and similar carbonaceous feed materials to produce methane and is particularly concerned with a catalytic gasification process carried out in the presence of a carbon-alkali metal catalyst.
2. Description of the Prior Art
Conventional processes for the manufacture of synthetic fuels by the gasification of coal or other carbonaceous solids generally require the reaction of carbon with steam, alone or in combination with oxygen, at temperatures between about 1200.degree. and about 2500.degree. F. to produce a gas which may contain some methane but consists primarily of hydrogen and carbon monoxide. This gas is subsequently reacted with additional steam to increase the hydrogen-to-carbon monoxide ratio by means of the water-gas shift reaction. Following this, the gas is usually treated to remove carbon dioxide and sulfur compounds and then fed to a catalytic methanation unit for reaction of the carbon monoxide and hydrogen to produce methane and water. It has been shown that processes of this type can be improved by carrying out the initial gasification step in the presence of a catalyst containing an alkali metal constituent. The alkali metal accelerates the steam-carbon gasification reaction and thus permits the generation of synthesis gas at somewhat lower temperatures than would otherwise be required or alternatively permits the use of a smaller reactor than might be necessary in the absence of such a catalyst. It is also known to carry out the water-gas shift reaction in the presence of such a catalyst.
The methanation reaction is conventionally carried out in the presence of a catalyst containing iron, nickel or cobalt as the principal constituent. Although catalysts of this type are reasonably effective, experience has shown that most such catalysts are highly sensitive to sulfur compounds and are quickly poisoned. To avoid this, the synthesis gas fed to the methanation unit may be treated to remove organic and inorganic constituents containing sulfur. This is generally done by first scrubbing the gas stream with a solvent such as monoethanolamine to eliminate most of the hydrogen sulfide, mercaptans, and the like and then removing the last traces of these impurities by adsorption on reduced zinc oxide or a similar adsorbent. Periodic regeneration of the methanation catalyst by treatment with hydrogen is generally required. These gas feed purification and catalyst regeneration steps are expensive. Moreover, the overall program is costly because of the large quantities of heat required in order to sustain the steam-carbon reaction, the large losses of low level exothermic heat of reaction from the water-gas shift and methanation reactions, and the substantial investment in equipment necessary to carry out the individual steps of the process.
To avoid difficulties of the type described above, it has been proposed that the steam-carbon reaction be used for the generation of a synthesis gas containing hydrogen and that the water-gas shift and methanation steps be eliminated by reacting the hydrogen thus produced with elemental carbon to form methane. It is known that the gasification of carbon with steam is an endothermic reaction and that the hydrogenation of carbon to form methane is exothermic. The heat liberated by the exothermic reaction of hydrogen with carbon to produce one mole of methane is about 83% of that necessary to generate the corresponding quantity of hydrogen by the endothermic reaction of steam with carbon. It has therefore been suggested that these two reactions be integrated to obtain an overall process which can be represented by the following equations: EQU C + 2H.sub.2 O .fwdarw. 2H.sub.2 + CO.sub.2 + 39,000 Btu (1) EQU C + 2H.sub.2 .fwdarw. CH.sub.4 + 32,570 Btu (2) EQU 2C + 2H.sub.2 O .fwdarw. CO.sub.2 + CH.sub.4 - 6570 Btu (3)
It has been proposed that these reactions be carried out simultaneously in parallel fluidized bed reaction zones. Ine one such reaction zone, feed coal would be reacted with recycle hydrogen at a pressure of 40 atmospheres and a temperature of 1500.degree. F. or higher to produce methane. In the other such vessel, char withdrawn from the hydrogenation zone would be reacted with steam at 40 atmospheres and 1200.degree. F. or higher to produce hydrogen and carbon dioxide. The gases from the hydrogenation zone would be treated for the recovery of methane and the remaining hydrogen would be recycled to the hydrogenation zone. Similarly, the gases from the steam gasification zone would be treated to remove carbon dioxide and the remaining hydrogen would be recycled to the hydrogenation zone. It has been said that the use of sodium carbonate as a catalyst in such a system is effective for the steam gasification reaction and will also help to increase the formation of methane in the hydrogenation reaction. The additional heat required to balance the system would be supplied by injecting oxygen into the steam gasification zone with the steam.
It has been said that a process of the type described above might conceivably be carried out in a single reaction vessel by introducing steam and recycle hydrogen into the vessel alternately. In an operation of this type, the injection of hydrogen would be commenced at a bed temperature of about 1500.degree. F. and continued until the carbon temperature reached about 1700.degree. F. At this point, the injection of hydrogen would be terminated and steam would be injected until the bed temperature had decreased from 1700.degree. F. to about 1500.degree. F. Temperatures of 1500.degree. F. or higher are apparently required to achieve satisfactory carbon-hydrogen reaction rates. Again, tonnage oxygen would be added to the injected steam to burn carbon and supply the added heat necessary to balance the system. It has also been suggested that such a process might be carried out by mixing the steam and recycle hydrogen in related proportions and injecting them into the bed at temperatures between 1500.degree. F. and 1700.degree. F. so that the reaction of steam with carbon to produce hydrogen and the reaction of hydrogen with carbon to produce methane could take place simultaneously. Four moles of hydrogen would be recycled for each mole of methane produced and tonnage oxygen would be supplied to provide the necessary heat.
Noncatalytic processes which seek to integrate the steam-carbon and carbon-hydrogen reactions in a manner similar to that described above have been studied extensively. It has been stated in the literature that such a process would become increasingly endothermic as the reaction temperature increases and less endothermic with increases in the hydrogen-to-steam ratio in the gasifier. At high hydrogen-to-steam ratios, the process would allegedly approach thermal balance but require that substantial quantities of excess hydrogen be supplied from an external source. At low hydrogen-to-steam ratios, it has been said that sufficient hydrogen to satisfy the hydrogen requirements might be produced but that large amounts of heat would have to be supplied to this system. This imbalance between thermal and hydrogen requirements has been said to prevail at all possible gasification temperatures, even in those cases where the amount of available hydrogen is increased by including a water-gas shift step downstream of the gasifier.
It has been said that a process of the type referred to above would be thermodynamically feasible if operated at a temperature between about 1700.degree. and 1800.degree. F. and at a pressure of about 200 atmospheres and at a relatively low hydrogen-to-steam ratio so that sufficient hydrogen to keep the system in hydrogen balance would be produced and that the heat deficiency could be made up by preheating the reactants to high temperatures, by providing internal heat exchange with a high temperature fluid, or by partial combustion of coal within the gasifier. The first two alternatives would appear to be of questionable practicality because of the large amount of heat that would have to be supplied and the need for rapid heat transfer to make the furnishing of this heat feasible. The injection of oxygen into the gasifier would permit the generation of heat at the point where it is needed, alleviate the heat transfer problem, and at the same time provide a high temperature zone in which part of the inlet steam could be rapidly decomposed to obtain higher steam conversion rates than might otherwise be achieved. The use of oxygen in this manner would have disadvantages, however, because it could add significantly to the cost of the process, make operation of the gasifier more difficult, and reduce the overall process efficiency.
Although a process of the type outlined above would, if feasible, have pronounced advantages over conventional processes requiring separate shifting and downstream methanation of the raw product gas, studies have shown that there are serious problems associated with any such process. The analyses of such processes have assumed, for example, that the carbon-hydrogen reaction is in equilibrium but it can be shown that such will not be the case in a practical system. Published data on the relative rates of various reactions in a noncatalytic steam-carbon-hydrogen system have shown that this reaction proceeds at only about one-thousandth of the rate of the steam-carbon reaction. Moreover, it has been pointed out that an effective system of this type would require the use of two separate vessels, one for the hydrogenation reaction and a second immediately below the first for gasification of the residual carbon with steam and oxygen. As a result of these and other difficulties, it has been concluded in the literature that present-day gasification technology is inadequate to permit the conversion of steam and coal into methane and carbon dioxide in a single step and that a reaction sequence involving (1) steam gasification and the combustion of coal with oxygen as an initial step, (2) the water-gas shift reaction as an intermediate step, and (3) the downstream methanation of carbon monoxide as a final step must be employed.